In certain paint spraying equipment, the paint is atomized and electrostatically charged to a high voltage relative to the workpiece being painted. The charge on the paint droplets attracts the droplets to the workpiece to greatly increase the paint transfer efficiency and to improve the uniformity of the applied coating. DC voltages on the order of 40,000 to 100,000 volts or more may be used for charging the paints. Normally, the workpiece is maintained at ground potential during painting. Since 100,000 volts will jump a several inch gap, care must be taken to keep the spray gun away from grounded objects while the high voltage is on to prevent arcing. Many paints have flammable solvents. Consequently, an arc from the high voltage paint charging circuit can result in an explosion and/or a fire. Also, the high voltage represents a hazard to any operating personnel in the vicinity of the spray gun.
Various techniques have been used for controlling arcing from an electrostatic power supply. In many electrostatic spray guns, a very large value resistor is placed in the high voltage circuit. During normal operation, the current flow in the high voltage circuit is very low and consequently the voltage drop across the resistor is not significant. As the output current increases when a grounded object is approached, there is a reduction in the output voltage due to the voltage drop across the resistor. The voltage drop reduces but does not eliminate the risk of arcing. The resistor also can reduce the amount of energy available in the event that arcing takes place. This in turn reduces the risk of a dangerous level shock to operating personnel and also reduces the risk that an arc will have sufficient energy to produce ignition of flammable paint solvents. Preferably, the paint charging electrode is small in mass and the resistor is placed very close to the charging electrode to minimize the capacitance between the high voltage paint charging electrode and ground, thereby minimizing the stored energy available in the event of arcing. For a rotary paint atomizer which is mounted in a spray booth, a stiff or substantially constant high voltage is generally used to achieve better coating quality. In other words, there is no current limiting resistor in the high voltage circuit. However, such systems may incorporate incipient arc detecting circuitry which detects an incipient arc event and interrupts the high voltage prior to actual arcing.
An early arc prevention technique involved looking at the magnitude of the DC current in the ground return for the high voltage circuit. When the current exceeded a predetermined maximum level, the high voltage was interrupted. However, if the output current suddenly increased, an arc could occur before the high voltage was interrupted. The arc would have sufficient energy to cause ignition. In the flammable atmosphere present in many spray booths during painting, such an arc cannot be tolerated. Unless the predetermined maximum current is established at a level significantly below the current required for an arc, the high voltage will not be interrupted prior to arcing. However, setting the predetermined maximum current at such a low level reduces the effectiveness of the power supply.
More recently, circuits have been developed to analyze the current in the high voltage ground return path. One technique looks at the rate of change in the high voltage DC current. As a grounded object is approached by the paint charging electrode, there is an increase in the DC current. When the rate of change or di/dt of the DC current exceeds a predetermined level, it is assumed that an arc is about to take place and the high voltage is interrupted prior to actual arcing. Thus, incipient arcing is detected. A corona discharge from the high voltage paint charging electrode prior to arcing also creates a low level AC current which is superimposed on the DC current. According to another technique, the magnitude of any AC current component superimposed on the DC current is detected. When the AC current component exceeds a predetermined level, it is determined that an incipient arcing condition is present and the high voltage is interrupted prior to actual arcing.
In prior art electrostatic power supplies, it has been necessary either to disable the incipient arc detection circuitry during start up to prevent automatic shutdown from the initial current change. If a grounded object is too close to the high voltage circuit during start up, an arc will occur. The prior art arc prevention circuits interrupt the high voltage whenever either a excessive ground return current is detected or an incipient arcing condition is detected. However, once the high voltage is interrupted, the systems required a manual reset by a systems operator. If, for example, a rotary atomizer is used to paint an automobile body on a production line, the quality of the paint applied to the body would be unacceptable if the high voltage is interrupted more than a brief time. It has been determined that if the voltage is reestablished within a short time, for example, of no more than about 1 second, an acceptable coating can be applied. However, the prior art arc preventing power supplies are not capable of automatically restarting after shutdown. If the power supply was quickly restarted and an arcing condition still existed, the resulting arc could cause a fire or an explosion.